I know, that the sum of the 3 angles of a triangle, is 180 degrees. But what about a square,pentagon,hexagon..?

You can solve this problem for yourself by dividing your shape into triangles.A square (or any quadrilateral) can be divided into 2 triangles, a pentagon into 3 triangles, a hexagon into 4 triangles, and so on. An n-sided polygon can be divided into n-2 triangles. The sum of the angles of each triangle is 180 degrees, so the sum of the angles of the n-sided polygon is 180*(n-2) degrees.